The present invention relates to methods and apparatus for transmitting selected control and alarm information in the form of high frequency signals using exsting refrigeration system defrost heater conductors as a transmission medium.
In the prior art, conveying selected control or alarm information to locations remote from sensors located within a refrigeration compartment have necessitated conductors be installed between the sensor and the remote location. In large commercial installations, as the number of functions to be monitored increases, the number of conductors increases with installation cableways within which to install the conductors. Further, if additional sensors are to be installed in existing units, space may be limited thus complicating installation of the conductors.
Also in the prior art, various methods must be utilized to periodically reduce the frost build-up on the evaporator coils. Otherwise, the efficiency of the refrigeration system would be decreased by the increasing accumulation of frost. The most common method used with a conventional refrigeration system incorporates a timer mechanism to control a defrost heater power contactor. Typically, the timer mechanism is positioned in proximity to the contactor, which in some refrigerator installations may be located up to 200 feet away from the evaporator section. The timing mechanism periodically applies power to the solenoid coil of the power contactor, closing the switch contacts therein. As the switch contacts are closed, power is applied to the defrost heaters located adjacent to the evaporator coils, with the heat thus generated reducing the frost build-up on the coils. A predetermined time period later, the timer mechanism removes power from the solenoid coil, deenergizing the power contactor and removing power from the defrost heaters.
As power is applied to the defrost heaters, approximately ninety percent of the energy generated by the heaters is absorbed by surrounding structures, such as the refrigeration compartment enclosure, the evaporator coils and the material which is being cooled. Accordingly, only about ten percent of the energy radiated from the heaters is absorbed by and used to reduce the frost build-up on the evaporator coils. During each defrost cycle the defrost heaters are energized and the refrigeration compressor is shut off which results in a waste of most of the electrical power applied to the defrost heater coils, since ninety percent of the dissipated heat is absorbed by the surrounding evaporator and evaporator coil structure without effecting frost removal. In addition, since the refrigeration cycle is shut down, the additional heat added to the evaporator must later be removed b y the refrigeration system, causing added wasting of electrical power during extended refrigerating cycles. Thus, it may be seen that operation of the defrost heaters, although necessary to reduce frost build-up on the evaporator coils, interferes with the proper operation of the refrigeration system. Accordingly, use of the above-described timer mechanism to control power to the defrost heaters is, at best, inefficient since the operation of the mechanism bears no relation to the actual conditions in the system evaporator coil section. These longer compressor and defrost heater operating times result in excess electrical power usage.
More recently, devices designed to be positioned adjacent the evaporator coils and which are used to detect the frost build-up have become available. These devices or sensors produce a signal functionally related to the accumulation of frost on the evaporator coil. This signal is used to initiate a defrost cycle directly related to the accumulation of frost, that is, a "demand" defrost cycle. The use of these devices generally requires that the signal functionally related to the frost build-up be generated by the device and transmitted over electrical conductors supplied for the purpose. The conductors supplied terminate at a timing mechanism similar to that above-described, but which is energized by the transmitted signal. The output of the detection device is functionally related to the build-up of the frost and set to provide a threshold signal which will energize the contactor at a preselected thickness of frost build-up. If such a device is utilized with a new installation, the electrical signal conductors may be positioned during construction. However, in both existing and planned commercial installations, such as the large, commercial refrigeration systems in a super market, the timer mechanism, power main and contactor may be located as much as 200 feet away from the evaporator section. Thus, the additional cost of materials and labor required in installing the additional conductors will be related to the distance between the timer mechanism and the evaporator section. Additionally, if such a device is to be retrofitted on an existing system, the cost of installing the conductors and of materials generally will be higher. Further, in an existing system, space may be limited, thus complicating installation of the conductors.
The present invention overcomes the difficulties of the prior art by providing a method and apparatus for generating high frequency signals containing selected control and alarm information and utilizing the electrical conductors associated with the defrost heaters as a transmission medium for the signals. The apparatus may be either retrofitted to an existing system without requiring additional cabling or readily utilized during the construction of a new refrigeration system.